Physical Sensors

Physical sensors used at physical measurenment. Physical measurands include temperature, strain, force, pressure, displacement, position, velocity, acceleration, optical radiation, sound, ﬂow rate, viscosity, and electromagnetic ﬁelds. Referring to Table 56.1, all but those transduction mechanisms listed in the chemical column are used in the design of physical sensors. Clearly, they comprise a very large proportion of all sensors. It is impossible to illustrate all of them, but three measurands stand out in terms of their widespread application: temperature, displacement (or associated force), and optical radiation.

Physical and Chemical Transduction Principles

Type Of Physical Sensors

Temperature Sensors

Temperature is an important parameter in many control systems, most familiarly in environmental control systems. Several distinctly different transduction mechanisms have been employed. The mercury thermometer was mentioned in the Introduction as a nonelectrical sensor. The most commonly used electrical temperature sensors are thermocouples, thermistors, and resistance thermometers. Thermocouples employ the Seebeck effect, which occurs at the junction of two dissimilar metal wires. A voltage difference is generated at the hot junction due to the difference in the energy distribution of thermally energized electrons in each metal. This voltage is measured across the cool ends of the two wires and changes linearly with temperature over a given range, depending on the choice of metals. To minimize measurement error the cool end of the couple must be kept at a constant temperature, and the voltmeter must have a high input impedance.

The resistance thermometer relies on the increase in resistance of a metal wire with increasing temperature. As the electrons in the metal gain thermal energy, they move about more rapidly and undergo more frequent collisions with each other and the atomic nuclei. These scattering events reduce the mobility of the electrons, and since resistance is inversely proportional to mobility, the resistance increases. Resistance thermometers consist of a coil of ﬁne metal wire. Platinum wire gives the largest linear range of operation. To determine the resistance indirectly, a constant current is supplied and the voltage is measured. A direct measurement can be made by placing the resistor in the sensing arm of a Wheatstone bridge and adjusting the opposing resistor to “balance” the bridge, which produces a null output. A measure of the sensitivity of a resistance thermometer is its temperature coefﬁcient of resistance: TCR = (ΔR/R)(1/ΔT) in units of % resistance per degree of temperature.

Thermistors are resistive elements made of semiconductor materials and have a negative coefﬁcient of resistance. The mechanism governing the resistance change of a thermistor is the increase in the number of conducting electrons with an increase in temperature due to thermal generation, i.e., the electrons which are the least tightly bound to the nucleus (valence electrons) gain sufﬁcient thermal energy to break away and become inﬂuenced by external ﬁelds. Thermistors can be measured in the same manner as resistance thermometers, but thermistors have up to 100 times higher TCR values.

Physical Sensors For Displacement And Force

Many types of forces are sensed by the displacements they create. For example, the force due to acceleration of a mass at the end of a spring will cause the spring to stretch and the mass to move. Its displacement from the zero acceleration position is governed by the force generated by the acceleration (F = m.a) and the restoring force of the spring. Another example is the displacement of the center of a deformable membrane due to a difference in pressure across it. Both of these examples use multiple transduction mechanisms to produce an electronic output: a primary mechanism which converts force to displacement (mechanical to mechanical) and then an intermediate mechanism to convert displacement to an electrical signal (mechanical to electrical).

Displacement can be measured by an associated capacitance. For example, the capacitance associated with a gap which is changing in length is given by C = area · dielectric constant/gap length. The gap must be very small compared to the surface area of the capacitor, since most dielectric constants are of the order of 1 x 10-13 farads/cm and with present methods, capacitance is readily resolvable to only about 10-12 farads. This is because measurement leads and contacts create parasitic capacitances the same order of magnitude. If the capacitance is measured at the generated site by an integrated circuit (see Section III), capacitances as small as 10-13 farads can be measured. Displacement is also commonly measured by the movement of a ferromagnetic core inside of an inductor coil. The displacement produces a change in inductance which can be measured by placing the inductor in an oscillator circuit and measuring the change in frequency of oscillation.

The most commonly used force sensor is the strain gage. It consists of metal wires which are stretched in response to a force. The resistance of the wire changes as it undergoes strain, i.e., a change in length, since the resistance of a wire is R = resistivity · length/cross-sectional area. The wire’s resistivity is a bulk property of the metal which is a constant for constant temperature. For example, a strain gage can be used to measure acceleration by attaching both ends of the wire to a cantilever beam, with one end of the wire at the attached beam end and the other at the free end. The cantilever beam free end moves in response to an applied force, such as the force due to acceleration which produces strain in the wire and a subsequent change in resistance. The sensitivity of a strain gage is described by the unitless gage factor, G = (ΔR/R)/(ΔL). For metal wires, gage factors typically range from 2 to 3. Semiconductors are known to exhibit piezoresistivity, which is a change in resistance in response to strain which involves a large change in resistivity in addition to the change in linear dimension. Piezoresistors have gage factors as high as 130. Piezoresistive strain gages are frequently used in microsensors.

Physical Sensors For Optical Radiation Measurentment

The intensity and frequency of optical radiation are parameters of growing interest and utility in consumer products such as the video camera and home security systems and in optical communications systems. The conversion of optical energy to electronic signals can be accomplished by several mechanisms (see radiant to electronic transduction in table above); however, the most commonly used is the photogeneration of carriers in semiconductors. The most often-used device is the p-n junction photodiode (Section III). The construction of this device is very similar to the diodes used in electronic circuits as rectiﬁers. The diode is operated in reverse bias, where very little current normally ﬂows. When light is incident on the structure and is absorbed in the semiconductor, energetic electrons are produced. These electrons ﬂow in response to the electric ﬁeld sustained internally across the junction, producing an externally measurable current. The current magnitude is proportional to the light intensity and also depends on the frequency of the light. The following figure shows the effects of varying incident optical intensity on the terminal current versus voltage behavior of a p-n junction. Note that for zero applied voltage, a net negative current ﬂows when the junction is illuminated. This device can therefore also be a source of power (a solar cell).